This invention relates to the separation of suspended particles from a fluid by the application of a magnetic field. In particular, the invention involves the use of magnetic separation techniques for the separation of mineral matter from carbonaceous fluids derived from oil shale.
Bituminous sedimentary rocks such as oil shale and alum shales represent a significant reserve of mineral raw materials from which energy can be recovered. These rocks have a predominantly fine grain structure and contain in the interstices between the grains valuable constituents in the form of bituminous residues which may also be of extremely fine grain form. The shales may also contain other inorganic constituents of greater or lesser value in the form of different minerals. The organic constituents of oil shales and alum shales are generally designated kerogen, which is a mixture of stabilized dry and solidified hydrocarbons produced by the sedimentation of organic substances. The predominant inorganic minerals are quartz, dolomite, albite, calcite and ankerite, which contain the metals silicon, aluminum, iron, magnesium, calcium and potassium. Iron minerals present are pyrite, ankerite, siderite, and pyrrhotites. Other metals such as uranium, copper, nickel, cobalt, palladium and molybdenum are also present, for example, as sulfides, silicates, and phosphates.
As used hereinafter, the term "oil shale" includes alum shale and other kerogenous shales. The term "mineral solids" refers to insoluble inorganic and organic particles which are present in carbonaceous fluids derived from oil shale and includes solids which have undergone changes in chemical composition as a result of the retorting or other steps in the shale recovery process. Retorted shale oil is the carbonaceous material (liquid or gaseous) which has been liberated from oil shale by volatilization as a result of a retorting process involving heating the oil shale to a temperature above about 370.degree. C., preferably 450.degree. C.-550.degree. C.
The retorting of oil shale can be carried out underground (in situ) or in above-ground retorts. The retorting can be aided by a stripping gas, such as steam, which is passed through the heated shale to facilitate removal of the retorted carbonaceous product. Retorted shale oil can contain a substantial quantity of mineral solids, including significant quantities of fines smaller than 100 micrometers in diameter. After large particles are removed by conventional filtration, cyclone separators, etc., the carbonaceous solids-lean effluent contains mineral particles predominantly smaller than 10 micrometers in diameter. The mineral solids in the effluent (which can be the feed to a subsequent separation step) can have a particle distribution of about 90 percent by weight of the particles smaller than 10 micrometers in diameter (equivalent sphere diameter) and more than 50, 70, or even 90 percent by weight smaller than 5 micrometers in diameter. In some cases, approximately 20 percent of the particles are smaller than 1 micrometer in diameter. The small size of the shale mineral particles has presented a difficult separations problem to the oil shale industry.
Magnetic processes have been used for recovery of strongly magnetic particles in other industries. See, for example, Watson et al, "A Superconducting Magnetic Separator and its Application in Improving Ceramic Raw Materials", Eleventh International Mineral Processing Congress 1975, University of Cagliari, Italy. Magnetic separation of ferromagnetic and paramagnetic particles from fluids involves exposing a suspension of particles in a fluid to a magnetic field to cause the migration of particles under the influence of the field (due to the field gradient) thereby permitting recovery of a fluid product having a reduced solids concentration. Of recent interest is the technique known as high-gradient magnetic separation (HGMS). HGMS involves the interaction between a filtration element comprised of a ferromagnetic material such as wire filaments and small ferromagnetic or paramagnetic particles in an applied magnetic field, i.e., a magnetic field provided by a source external to the ferromagnetic element. Magnetic field gradients around the filaments are several orders of magnitude higher than in the absence of the ferromagnetic filtration element. The fluid feed stream containing suspended particles is passed in the vicinity of the ferromagnetic element. Those magnetic particles which pass within the capturing distance that the element presents to the fluid stream are caused to migrate to the element and are removed from the stream. In commercial practice the ferromagnetic element is in the form of a steel mesh and the external magnetic field is generally applied by an electromagnet. Superconducting electromagnets and permanent magnets have also been proposed for this application. An example of a HGMS system suitable for use according to this invention is described in the article "New Tasks For Magnetism". Chemical Engineering, Jan. 7, 1974, pp. 50-52, which is incorporated herein by reference.
The applicability of magnetic separation techniques for removal of solids is dependent on a number of complex phenomena. Though a number of transition metals can be ferromagnetic or paramagnetic, the magnetization of particles containing the metals is strongly related to the chemical and morphological form of the metal. When mineral solids are to be separated, the distribution of the magnetic material among the particles is a limiting factor to the separability. For example when magnetic separation is applied to a flowing fluid, the magnetic force must overcome both gravitational forces as well as fluid drag forces. The resultant force, then, is related to the size and density of the particles relative to the amount of magnetic material present in each particle. If sufficient magnetic material is not present in most of the individual particles, poor separation will result regardless of the total amount of magnetic material present.
Much effort has been directed toward the study of magnetic separation of solids from coal liquefaction products. See, for example, "Magnetic Separation of Mineral Matter from Coal Liquids", EPRI AF-508 (August 1977) and EPRI AF-875 (November 1978) available from the Electric Power Research Institute, 3412 Hillview Ave., Palo Alto, Calif. 94304. It is reported in EPRI AF-508, Section 365-1, page 4.1, that under proper operating conditions, magnetic separation could remove about 99 percent inorganic sulfur and about 40 percent mineral ash from coal liquids at optimum temperature. Other applications of magnetic separation to coal-derived liquids are reported in U.S. Pat. No. 3,725,241, issued Apr. 3, 1973 to Chervenak for "Solids Removal From Hydrogenated Coal Liquids", and U.S. Pat. No. 3,976,557, issued Aug. 24, 1976 to Shen et al for "Pretreatment of Coal-Derived Liquid to Improve Magnetic Separation of Solids" and the background references identified therein.
The application of magnetic separation to coal-derived liquids has been motivated by the large amount of iron present in coal ash. The mineral matter present in oil shales, however, is not rich in iron. A study of magnetic properties of shales was reported by Noltimer et al., in "Thermomagnetic Study of Coal and Associated Roof Shale", IEEE Transaction on Magnetics, Vol. MAG-12, No. 5, September 1976, pages 528-531.